Biotite
Biotite is a common rock-forming mica, a potassium iron magnesium aluminosilicate distinguished by perfect basal cleavage that splits it into thin, flexible, dark sheets. It crystallizes in the monoclinic system and ranges from dark brown to black, sometimes with a bronze or greenish cast. As one of the most widespread minerals in igneous and metamorphic rocks, biotite serves geologists as a key indicator of a rock's cooling history and metamorphic grade.
Biotite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | K(Mg,Fe)3AlSi3O10(OH)2 |
| Mohs hardness | 2.5–3 |
| Crystal system | Monoclinic |
| Luster | Vitreous to pearly (splendent on cleavage) |
| Streak | White to gray |
| Cleavage | Perfect basal {001}, one direction (micaceous) |
| Fracture | None; flexible, inelastic cleavage flakes |
| Specific gravity | 2.7–3.4 |
| Transparency | Translucent in thin sheets, opaque in mass |
| Common colors | Black, Dark brown, Dark green |
| Rarity | common |
Geology & background
Biotite crystallizes directly from cooling magma in granites, diorites, and pegmatites, and also grows during regional metamorphism of shales and mudstones, where it often defines the foliation seen in schists and gneisses. Because its potassium-argon composition locks in at a known closure temperature, geologists use biotite crystals to date the cooling of metamorphic terrains, giving it importance well beyond its modest looks. Large books of biotite crystals, sometimes reaching a foot across, have been recovered from pegmatites in Ontario, Madagascar, and parts of New England.
Rockhounds usually find biotite as loose, dark flakes scattered through granite or as larger books within pegmatite pockets, and thin sheets can be peeled apart with a fingernail to reveal an elasticity that springs the mineral back after bending rather than leaving a crease. Because biotite weathers more quickly than quartz and feldspar, it often appears altered to a golden, chlorite-rich mineral on older or weathered specimens. Fresh, unweathered flakes with sharp, mirror-like cleavage faces are the most desirable for teaching mineral identification.
How to identify Biotite
Biotite appears black to dark brown or greenish-black, with a distinctive vitreous to pearly luster on its cleavage faces. Its hardness is low, around 2.5 to 3, easily scratched with a fingernail, and its streak is white to pale gray. The defining test is its perfect basal cleavage, which allows thin sheets to peel off in flexible, elastic layers that spring back after bending, unlike brittle chlorite, which it superficially resembles. Biotite is often mistaken for muscovite, but muscovite stays colorless to pale, while biotite is consistently dark.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Biotite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Biotite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Thin sheets of biotite can be split so fine that early scientists used them as windows in high-temperature furnace viewports.
- Radioactive trace elements trapped inside some biotite crystals create tiny dark halos around microscopic zircon inclusions, visible under a microscope.
- Because biotite's cleavage flakes are elastic rather than merely flexible, geologists use a simple bend-and-release test to identify it in the field.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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